1. Field of the Invention
This invention is generally related to the field of semiconductor devices, and, more particularly, to an optical isolator device, and various methods of making same.
2. Description of the Related Art
In many practical applications, it is desired to transmit signals between two electrical circuits that are electrically isolated from each other. Due to a desire for small size and complete electrical isolation between the circuits, four terminal devices, commonly called opto-isolators, have been developed. These devices utilize optical coupling, rather than the electrical coupling used in transformers and relays, to link the two electrical circuits. Opto-isolators use a light source, commonly a light emitting diode (LED), located in the electrical input circuit, and a photodetector or receiver, located in the electrical output circuit and optically coupled to the light source, to couple the two electrical circuits. Current flowing in the input circuit causes the LED to emit light, and some of this light is received by the photodetector and causes an electrical current to flow in the output circuit. It should be understood that the term “light,” as used in the specification, refers to electromagnetic radiation in both the visible and infrared regions.
Opto-isolators typically use discrete devices. That is, the light source and photodetector are manufactured separately and individually positioned in an optical cavity to form the opto-isolator. The light source is connected to two input terminals, and the photodetector is connected to two output terminals. The light source and photodetector are generally formed from different materials. For example, opto-isolators commonly used today have LEDs made from direct semiconductors, such as GaAs, GaP, SiC, GaAs1-xPx and Ga1-xAlxAs photodetectors. The presence of discrete devices means that considerable care has to be exercised in positioning the LED and the photodetector, both with respect to each other and the cavity, to obtain efficient light coupling. Additionally, cavity construction and the material used to form the cavity are often critical.
For reasons of manufacturing economics, as well as efficient coupling of light between the light source and detector, a monolithic or integrated opto-isolator would be desirable. Such a device would be fabricated on a single semiconductor chip from a single semiconductor material, i.e., both the light source and light detector would consist of the same semiconductor material. The use of a single material and a single chip affords the possibility of simplicity of fabrication as positioning of the LED and photodetector with respect to each other may be accomplished automatically. The automatic positioning of the LED and photodetector with respect to each other may reduce optical losses due to misalignment of the LED and photodetector.
U.S. Pat. No. 4,275,404 discloses one illustrative example of a monolithic opto-isolator. As depicted therein, the opto-isolator is constructed with the light source and the light detector disposed on the substrate formed from an epitaxially grown Group II–VI or Group III–V semiconductor compound and admixtures thereof. According to the disclosure, U.S. Pat. No. 4,275,404 contemplates the use of a substrate having a resistivity of at least 104 ohm-cm to obtain high isolation voltages.
Despite the existence of optical isolators in the prior art, there is still a desire for such opto-isolators that may be fabricated economically and reliably and constructed on a single chip or substrate. Such a construction would allow easy integration with other manufacturing processes, reduced costs, increased packing densities and reduce the overall size of a resulting product incorporating such a device.
The present invention is directed to a device and various methods that may solve, or at least reduce, some or all of the aforementioned problems.